Can field curvature change over the focus range? I recon it can.
I remember a discussion on here with a fellow ultra micro Nikkor 55mm f2 owner, and we found that increasing the mounting distance from 1:3 magnification showed rapidly increasing field curvature.
So it would stand to reason that some WA lenses also can show quite some field curvature at MTF test distances but les so at "normal" focussing distances? A real world image would answer that (.